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Feb 8, 2012
Offset- is a term used to describe how far from center a wheel will meet a hub. A wheel that meets the hub directly in it's centerline, has an offset of 0. Anything out of center becomes negative, or positive, these are illustrated below. Adjusting offset will also influence backspace, defined below.

Positive offset- Causes a wheel to be tucked into the fender (Typically found in stock, lowered applications)
-Common figures include: +15,+18,+38
Negative offset- Causes a wheel to be pushed away from the fender (Typically found in lifted applications)
-Common figures include: -12,-24,-44

Making sense of Millimeters: Wheel offsets are measure in millimeters so to put things in perspective here are some rough conversions:

Backspace- is the amount of space between the wheels' mounting surface, and the inside edge of the wheel. Providing that you retain the backspacing of a wheel, when choosing new wheels, your suspension clearances remain the same. If you take off a stock wheel with a BS of 4.5", and replace it with an aftermarket wheel with the same backspacing, the rim will be provided adequate clearance, this does not however take into account your tires' width. In the event that you know the width of a wheel, but what to figure out the BS you can do so: A 20x9" wheel is truly 10" wide, putting the centerline at 4.5" provided the wheel has an offset of 0, now moving the offset to the inside, or outside changes the BS, a positive offset increases backspace, while a negative offset decreases backspace.

Rim Width- Width is measured inside of the lips of the wheel, a 20x10 wheel has 10" of mounting space, but the lips that the rim mount to take up 1/2" each, if you were to measure on the outside of the rim you'd find that it was really 11" wide.

Bolt Pattern- Refers to the number, and spacing of bolts in a wheel.
6 bolt GM pattern:6x5.5" (6x139.7)
8 lug GM pattern: 8x6.5" (8x165.1)
New 8 lug GM pattern:8x180

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